1. Field of the Invention
The invention relates to a method for producing a polarizing plate. The invention also relates to polarizing plate obtained by the above production method. The polarizing plate may be used alone or in the form of a laminate serving as an optical film to form an image display such as a flat panel display such as a liquid crystal display (hereinafter abbreviated as “LCD”) and an electroluminescence display (hereinafter abbreviated as “ELD”), and a plasma display panel.
2. Description of the Related Art
Polarizing plates for use in flat panel displays, especially for use in LCDs, generally use a polyvinyl alcohol-based film as a main raw material. Polarizing plates that are preferably used are produced by a process including the steps of stretching a polyvinyl alcohol-based film containing a dichroic material such as iodine so as to provide sufficient optical properties for LCDs and bonding a transparent protective film to the stretched film. The polyvinyl alcohol-based polarizers are produced through stretching and thus can easily shrink. The polyvinyl alcohol-based film uses a hydrophilic polymer and thus is very apt to be deformed especially under humidified conditions. The mechanical strength of the polyvinyl alcohol-based film itself is also low, and, therefore, there is a problem in which the film can be torn. Thus, there are used polarizing plates reinforced by bonding a transparent protective film to one or both sides of the polarizer. Such polarizing plates are produced by bonding the polarizer and the transparent protective film together with an adhesive. A polyvinyl alcohol-based adhesive, which is an aqueous solution, is generally used for bonding between triacetylcellulose and the polyvinyl alcohol-based polarizer.
In general methods for producing polarizing plates, transparent protective films identical to each other in properties such as physical properties and a thickness are bonded to both sides of a polarizer. In this case, the transparent protective films are arranged in a substantially line-symmetrical manner with respect to the polarizer placed at the center, and, therefore, even if the polarizer contracts, curling will not occur in any direction. In recent years, however, polarizing plates that are configured to include a polarizer and transparent protective films different in physical properties or thickness bonded to both sides of the polarizer have increased. In this case where the transparent protective films provided on both sides of the polarizer are different in physical properties or thickness from each other, the two sides of the polarizer are asymmetrical so that there is a problem in which curling toward one side can occur in the process of producing the polarizing plate. The curled polarizing plate has a problem in which air bubbles can be trapped in the process of bonding it to a liquid crystal cell. As the functionality, brightness and size of LCDs increase, polarizing plates for use in LCDs also have been required to have high handleability, and thus it is desired that curling of polarizing plates should also be suppressed.
When films are bonded, curling is generally controlled by balancing the tensions on the films being bonded to one another. In methods for producing polarizing plates, however, it is difficult to control curling only by the tension control, particularly in the case that transparent protective films different in thickness are used, because the polarizer and transparent protective films are immersed in water in the production process so that addition and removal of water are remarkable and thus swelling and shrinking of the films are caused. A pressure-sensitive adhesive layer protected by a separator is often provided on one side of a polarizing plate, and a surface protection film is often detachably provided on the other side. Alternatively, therefore, curling of a polarizing plate may be controlled by a method of controlling the tension in the process of placing the surface protection film. Although curling can be controlled by this method, however, curling of the polarizing plate itself is not directly controlled by this method so that when the polarizing is bonded to a liquid crystal cell, a force that tries to curl the polarizing plate can act to easily cause stress on the resulting panel.
As a method for suppressing curling when transparent protective films are simultaneously bonded in an asymmetrical manner to both sides of a polarizer to form a polarizing plate, there is proposed a method for producing a polarizing plate that includes: bonding a transparent protective film to one side of a polarizer under a controlled tension such that the transparent protective film can have a flat state after the bonding; and then bonding another transparent protective film to the other side under a controlled tension such that the transparent protective film can have a flat state after the bonding (see JP-A No. 2004-117482). In this method, however, swelling and shrinking of the films frequently occur due to addition and removal of water so that there is a possibility that control of curling is insufficient.